function studentTestNotLeader(){
    var jack = new Student(0002 , 'Jack' , 21 , new klass(2));
    var expectResult = 'My name is Jack. I am 21 years old. I am a Student. I am at Class 2.';
    (expectResult === jack.introduce()) ? console.log('studentTestNotLeader passed!') : console.log('studentTestNotLeader failed!');
}

function studentTestIsLeader(){
    var jack = new Student(0002 , 'Jack' , 21 , new klass(2));
    jack.klassObj.assignLeader(jack);
    var expectResult = 'My name is Jack. I am 21 years old. I am a Student. I am Leader of Class 2.';
    (expectResult === jack.introduce()) ? console.log('studentTestIsLeader passed!') : console.log('studentTestIsLeader failed!');
}
studentTestNotLeader();
studentTestIsLeader();